The present invention relates to a differential gear mechanism.
An example of prior art relevant to the present invention has been shown in U.S. Pat. No. 3,884,096, British Pat. No. 1,429,359, Italian Pat. No. 989,892 or West German Patent Laid Open Publication No. 2,334,043. In these prior art references, a differential case is provided which is driven from the outside and which rotates a pair of worm gears which are provided in the differential case and are each connected to different axle shafts, and a plurality of element gears which are composed of a worm wheel and spur gears are provided on both sides of the worm wheel. The worm gears are arranged coaxially and are each rotatable independently. The element gears are arranged in one set of two elements gears and a plurality of sets are provided around the worm gears. These element gears are supported by the differential case and are rotatable. For each pair of element gears, their spur gears engage with each other and their worm wheels engage with each of the worm gears.
The above-described differential gear mechanism compensates for a rotation difference between two axle shafts such as a conventional differential gear mechanism, and what is more, distributes a large drive torque to an axle shaft with slower rotation by automatically varying a distribution of torques to the axle shafts. That is, the differential gear mechanism transmits larger torque to an accelerator shaft with larger resistance than that of the other accelerator shaft with smaller resistance, according to certain torque bias ratio which is decided by a lead angle of the worm gears and the friction state during driving of the worm gears.
However, in such differential gear mechanism, when one wheel is disengaged and causes skidding to make resistance to one axle shaft zero, torque is not transmitted to the other axle shaft. Because of this, there was a problem that a vehicle could not get out of the skidding state unaided.